The invention relates to a method of compensating the interference DC voltages in the electrode circuit in magnetic-conductive flow measurement with periodically reversed DC magnetic field in which the useful signal is obtained by sampling and storing the signal voltage after each reversal of the magnetic field at opposite polarity values of said magnetic field during a sampling time interval and forming the difference of the stored sampled values and wherein in a compensating time interval following each sampling time interval a compensation voltage is produced by sampling and storing the signal voltage, which compensation voltage is superimposed oppositely on the signal voltage for compensating the signal voltage within the compensation time interval to the value zero and is retained until the next compensation time interval, and to an arrangement for carrying out the method.
A method of this type is known from U.S. Pat. No. 4,210,022. The difference of the sampling values obtained with opposite polarity values of the magnetic field gives a useful signal which is freed from constant interference DC voltages which as is known in the magnetic-inductive flow measurement can be as much as a thousand times greater than the useful signal. By the formation and superimposing of an additional compensation voltage linear time variations of the interference DC voltages between the successive sampling time intervals are also compensated and above all the amplifiers and subtraction circuits used to process the signal voltage are prevented from being overdriven due to the interference DC voltages building up slowly to a very high value.
In this known method each compensation time interval lies in a pause in the magnetic field which is inserted in each case between two successive partial periods in which the magnetic field assumes its oppositely poled value. The signal voltage sampled to form the compensation voltage is thus the pure interference voltage. The magnetic field must therefore be periodically switched between three values such that the pauses in the magnetic field and the compensation time intervals contained therein cannot be utilized for observing the flow.
In similar manner, in a method known from U.S. Pat. No. 4,010,644 a compensation of time variations of the interference DC voltages is carried out by forming a compensation voltage which is oppositely superimposed on the signal voltage. In this known method the magnetic field is switched to and fro between two different values, one of which may be zero. In this case in each partial period in which the magnetic field has the smaller value (or the value zero) two compensation time intervals lie at the start and the end of the partial period respectively, and between said two compensation time intervals there is a sampling time interval in which only the interference DC voltage change which has taken place since the last compensation is sampled and stored. On the other hand, in the other partial period no compensation takes place but only a sampling and storing of the compensated signal voltage which is equal to the sum of the useful signal and interference DC voltage change. This method is asymmetrical in time and the stored sampling values whose difference is formed have different orders of magnitude.
The problem underlying the invention is the provision of a method of compensating interference DC voltages in the magnetic-conductive flow measurement which permits the maximum possible utililzation of the available time for observing the flow and gives a large useful signal with respect to the power expenditure; the stored sample values subjected to the difference formation being of the same order of magnitude.
According to the invention this is achieved in that each compensation time interval lies within the time interval, corresponding to the switched-on magnetic field, in which also the preceding sampling time interval lies.
With the method according to the invention in completely regular manner in each partial period corresponding to the one or the other value of the magnetic field firstly a sampling and storing of the compensated signal voltage takes place and thereafter a compensation of the signal voltage to the value zero. Thus, the compensation voltage also includes the useful signal at the compensation instant. Since this compensation voltage in the sampling time interval of the following partial period remains superimposed on the signal voltage then obtaining, in each sampling time interval to form the stored sampling value a voltage is sampled which contains the sum of the useful signals in a compensation time interval and in the following sampling time interval. In this manner the compensation time interval is also used for obtaining the measured value signal and observing the flow. The stored sampling values whose difference is formed are of the same order of magnitude. This method is particularly suitable when the magnetic field is switched to and fro without pauses between two oppositely poled values so that no magnetic field intervals are lost for the observation of the flow.
An advantageous further development of the method of the invention resides in that at the start of each sampling time interval for a short time an instantaneous value sampling and storing of the signal voltage takes place without integration and the sampled signal voltage is applied to an integrating storage member only in the remaining part of the sampling time interval.
This further development substantially shortens the response time so that even very rapid flow changes are immediately detected and indicated.
A particular advantage of the method according to the invention resides in that by the nature of the signal formation the influence of an interference AC voltage superimposed on the useful signal can also be obviated without a defined relationship having to exist between the duration of the sampling time interval and the period of the interference AC voltage.
A preferred arrangement for carrying out this further development of the method for the inductive flow measurement of an electrically conductive liquid flowing in a conduit, comprising a magnetic field generator which generates a periodically reversed magnetic field passing through the conduit perpendicularly to the flow direction, two electrodes which are disposed in the conduit and are connected to the inputs of a measuring amplifier, sample and hold circuits which are connected to the output of the measuring amplifier and controlled by a control means in such a manner that they sample the output voltage of the measuring amplifier for equal induction values of opposite sign and store the sampling values until the next sampling, a circuit for forming the difference of the stored sampling values and a storing control circuit which is disposed in a closed loop between the output and the input of the measuring amplifier and which in each compensation time interval is connected to the output of the measuring amplifier, forms a compensation voltage value regulating the output voltage of the measuring amplifier to the value zero and maintains said compensation voltage value until the next compensation time interval, is characterized according to the invention in that each sample and hold circuit includes an integrating RC member to which the signal voltage is applied via a switch closed for the duration of the associated sampling time intervals, and that there is connected in parallel with the resistor of each RC member a switch which is briefly closed at the start of each of the associated sampling time intervals.